Nvidia Series 10XX on Mac OS
The GTX 10XX cards were not supported under OS X until just this past week. So benchmark data will be hard to find. For now.

Also, if you haven't already, study up on using newer Nvidia cards in the classic "cheesegrater" Mac Pro's, there are some caveats, especially if the card has not be reflashed. (off the shelf PC cards will work, but you won't have a startup splash and you may need to switch the monitor to a Mac card or use SSH to update the driver after installing OS updates)

Cycles can render across multiple cards, however the scene must copied in its entirety to each card, so you don't get to "add" the VRAM amounts. See: Does Blender support dual-GPU rendering if the two GPUs aren&#39;t the same?